The present invention relates to the art of radiation detection. It finds particular application in conjunction with CT scanners and will be described with reference thereto. It is to be appreciated, however, that the invention may find further application in other radiation detection devices.
In fourth generation CT scanners, a plurality of x-ray detectors have been stationarily mounted in a ring circumscribing the scan circle. X-ray energy emitted by the x-ray tube was directed to impinge upon a segment of detectors disposed on the opposite side of the scan circle. As the x-ray tube rotated around the scan circle, the irradiated portion of the stationary detector array shifted.
Typically, each detector included a scintillation crystal which converts x-ray energy into light energy. A silicon photodiode converted the light energy into an electric current. The scintillation crystal was normally a rectangular prism which is cut to match the rectangular photosensitive face of the photodiode.
Conventional photodiodes have lead wires connected to opposite ends of their photosensitive face. In order to obtain good optical coupling between the scintillation crystal and the photosensitive face, scintillation crystals terminated short of the lead wire connections. That is, the scintillation crystal has actually been smaller than the photosensitive face. This left a small length of photodiode which was not shielded by the scintillation crystal from the incident radiation. If any x-rays impinged upon the exposed photodiode surface, they tended to induce charges which migrated to the diode collector electrodes and contributed an undesirable component to the diode current.
A preamplifier amplified the photodiode current to produce a voltage signal indicative of the intensity of the radiation incident on the scintillation crystal. Of course, the performance of the CT scanner was dependent on how faithfully these components report the intensity of the incident radiation.
To optimize the results, the preamplifier should be selected such that its output is limited by the x-ray photon flux and not by electronic noise. To achieve this performance goal, preamplifier circuit designs commonly required a low capacitance and high resistance input. Because the capacitance dropped and the resistance increased as the photosensitive area of the photodiode decreased, optimal preamplifier performance called for a small photodiode.
However, other design criteria called for a large scintillation crystal, hence, large photosensitive diode face. More specifically, fourth generation scanners have been sensitive to transverse movement of the x-ray spot. The transverse focal spot movement was commonly due to wobbling of the rotating anode target, anode surface irregularities, or the like. The focal spot wobble caused corresponding periodic fluctuations in the x-ray tube output. These periodic fluctuations caused interference patterns to be superimposed on the CT image, known as "rotor ripple" artifacts. The transverse wobble of the x-ray spot tended to cause a like transverse wobble of the x-ray fan beam, shifting the beam in part off the scintillation crystals of the x-ray detectors. Elongated scintillation crystals enabled the full width of the x-ray beam to be received even during anode target wobble, hence, reduced wobble artifacts. However, elongating the scintillation crystal heretofore required elongating or enlarging the photosensitive diode face which increased its capacitance and reduced its resistance which, in turn, reduced the performance of the preamplifier. Thus, there has been a trade-off between amplifier noise signal degradation and rotor ripple signal degradation.
The present invention contemplates a new and improved detector design which overcomes the above referenced problems and others.